The invention relates to a motor-compressor unit comprising a hermetically sealed housing, a vibration motor having a rotationally vibrating drive shaft, and a symmetrical twin piston compressor comprising two oppositely disposed identical pistons, which can be reciprocated in respective cylinders by the drive shaft to influence a respective compression space, having cylinder end walls which each bound the respective compression spaces and which each comprise inlet and outlet valves.
Such a motor-compressor is known from U.S. Pat. No. 3,937,600.
A problem associated with motor-compressors of this type is "drifting" of the piston, i.e. the piston drifts away from its center position, causing the piston to run asymmetrically and the efficiency to be reduced.
This problem also occurs in refrigerating machines with a single free piston as described in U.S. Pat. No. 3,828,588. In these machines piston drift is caused by a leakage flow of the work medium along the piston from the compression space to a buffer space and vice versa. Drifting is then mitigated by means of a system of ducts which establish a connection between the compression space and the buffer space at specific instants.
U.S. Pat. No. 3,937,600 relates to a motor-compressor and provides a solution to preclude an excessive piston stroke as a result of a decreasing resonant frequency of the mass-spring system, which may lead to collision of the piston against the cylinder end wall (overstroke prevention). For this purpose the compressor is provided with a gas-spring cylinder at both ends. When the piston stroke becomes too large medium will flow from the surrounding to the relevant gas-spring cylinder via a duct system in an extreme position of a piston, thereby causing the stiffness of this gas spring to increase. As this results in a difference in stiffness of the two gas springs the pressures in the gas-spring cylinders are balanced in the piston centre position by temporarily interconnecting the gas-spring cylinders via ducts and the space within the double piston. The spring stiffness of the entire mass-spring system and hence the resonant frequency increases until a normal stroke length is attained again. This compressor is of an intricate construction and should be manufactured with a high accuracy in order to achieve the desired goal.
It is an object of the invention to minimise piston drifting in a simple way in a motor-compressor unit of the type defined in the opening paragraph.